The present invention relates generally to heat transfer elements and more particularly to a passive thermal spreader including at least one oscillating thermosyphon.
Continued growth in the density of electronic devices in general and in aircraft and spacecraft in particular gives rise to the need for innovative, miniature heat transfer elements capable of maintaining high heat flux operation in harsh environments ranging from the micro-gravity ( less than  less than 1 G) conditions in space to the extremely high acceleration loads encountered by modern military aircraft during evasive maneuvering (up to 10 G).
Conventional heat pipes generally include capillary wicks for inducing transport of the condensed working fluid from the cooler, condenser section, back to the heated, evaporator section. In this way, heat transport through the evaporation and condensation of the working fluid is sustained. Generally, the performance of conventional heat pipes is greatly affected by the gravitational field due to their reliance on wicking to complete working fluid circulation within the heat pipe device. This can be a significant limitation especially when operation in high acceleration loading conditions is desired.
An alternative to the capillary wick heat pipe is found in capillary pumped loop heat transfer devices. While these pumped heat transfer heat devices are capable of operation in a wider range of G forces, they are again disadvantaged by greatly reduced heat exchange capability during operation outside the 1 G window. Moreover, such devices are generally slow to respond to changing heat load conditions and since they must include a reservoir, and a wick structure, they are also not compact. The addition of the reservoir adds complexity, cost and weight. As can be seen, the conventional heat pipe and capillary pumped loop heat transfer devices in use today fail to provide effective high heat flux transfer across the range of operating conditions encountered by aircraft and spacecraft.
A need exists therefore for a heat transfer device capable of passive, yet high heat flux operation throughout a range of operating conditions from much less than 1 G to 10 G or greater (micro to high G). Such a device would be capable of providing high heat flux transfer across a wide range of operating conditions, be self contained and passive in operation.
Accordingly, it is a primary object of the present invention to provide a passive thermal spreader and method overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide a passive thermal spreader including internal, sealed nonconventional thermosyphons.
Yet another object of the present invention is to provide a passive thermal spreader capable of reliable, high heat flux operation in environments ranging from micro to high G.
Still another object of the present invention is to provide a passive thermal spreader that is self contained, requiring no external work input to maintain operation.
Another object of the present invention is to provide a passive thermal spreader having a heat transfer plate including two independent, sealed thermosyphons.
Still another object of the present invention is to provide a thermal spreader that is small in size, yet capable of removing heat greater than 150 Watts per square centimeter.
Yet another object of the present invention is to provide a passive thermal spreader having three heat transfer plates laminated together and a pair of thermosyphons defined within the interface of each pair of plates, respectively.
Additional objects, advantages and other novel features of the invention will be set forth, in part, in the description that follows and will, in part, become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention as described herein, a passive thermal spreader and method of fabrication includes a heat transfer plate formed from laminated plates including an internal sealed thermosyphon defined within the interface of the plates.
The preferred embodiment of the passive thermal spreader selected to illustrate the invention includes a pair of distinct thermosyphons defined within the interface of three laminated, stacked plates, designated upper, middle and lower, respectively. The first thermosyphon is defined within the interface of the upper and middle plates and the second thermosyphon is defined within the interface of the middle and lower plates. The pair of thermosyphons compliment each other during operation, assuring the desirable high heat flux heat transfer.
Each thermosyphon includes a plurality of channels arranged boustrophedonically, that is, crossing the plate in an alternating left-right then right-left or back and forth manner. Each of the channels includes an elongate straight portion and an end turn portion for connection to the adjacent channel elongate straight portion. A working fluid is incorporated into each of the thermosyphons to effect the heat transfer.
According to an important aspect of the present invention, each elongate straight portion includes a channel divider, which in the preferred embodiment is a thin wire. As will be described in more detail below, the thin wire directly contributes to the high performance of the thermal spreader of the present invention by assisting the transport of the working fluid. Contrary to the standard heat pipe which relies on a complete transfer of working fluid from the condenser to the evaporator through a capillary wick, it has been determined that the thermal spreader of the present invention requires only a portion of the working fluid to be pumped back to the evaporator though the sub channels formed by the thin wire. Thus, in the present invention, only a portion of working fluid is necessary to generate a vapor momentum driving force and to provide the desired high heat flux heat transfer.
More specifically, during operation of the thermal spreader of the present invention, a two phase liquid-vapor flow condition is created characterized by slugs of liquid separated by vapor. Each channel exhibits a pressure that is different from its adjacent channel and the vapor drives the slugs of liquid along the channels based upon the differential pressure of adjacent channels. In the case where no slugs of liquid exist in the evaporator within a given channel, the capillary force created by the thin wire will still pump a fraction of liquid into the evaporator. While the amount of the liquid reaching the evaporator may be very small, the vapor pressure becomes high enough to drive the liquid along the channel until it reaches an adjacent channel, thereby sustaining the desirable two phase flow condition across the thermal spreader. This completely avoids dry out conditions and facilitates the desirable high heat flux heat transfer operation. Moreover, by avoiding the conventional heat pipe requirement of complete liquid transport through the capillary wick, the high heat flux operation is assured across an extraordinarily broad range of G loads from the micro gravity environment encountered in space to the 10 G loads encountered by modern military aircraft when performing evasive maneuvers.